I have a 99 dodge ram 1500 5.2 liter that when i have the slightest load the A/C Ducts switch to Defrost.I have been told by a dodge mechanic that it is 1 of the 2 check valves.One of the check valves is located right out of the side of the manifold the other one is located in the vacuum line that goes into the cab of the truck on the firewall.I cant get the part # from my local dealer i just need the 2 part numbers